A hearth of a metallurgical furnace usually has an outer steel shell, typically with at least one taphole for tapping molten metal, and a lining of refractory material for containing the molten metal bath at high temperatures, in excess of 1100° C. The lining includes a lateral lining of the shell, hereinafter called wall lining, and a lining in the bottom of the hearth i.e. the hearth pad.
In the field of blast furnaces, there are various approaches for constructing a wall lining. In a well-known approach, the wall lining is a brickwork of multiple concentric rings of comparatively small bricks. These are typically made of high-conductivity hot-pressed carbon. Another approach uses comparatively larger blocks of refractory, typically also of carbonaceous material (including carbon, hot-pressed carbon, graphite, semi-graphite and hot-pressed semi-graphite). Usually, large blocks are installed in a single thickness reaching from the shell to the hot face so that the lining consists of the same material over its entire cross-section. A further known approach, which aims at increasing protection and durability of the wall lining, consists in providing an additional so-called ceramic cup including a lateral inner layer of high-melting point ceramics, e.g. high alumina content pre-cast blocs, for protecting the carbonaceous blocs of the wall lining. Also well-known are hearth configurations with a composite lining of two annular layers of different materials. Usually, materials are used so that thermal conductivity of the outer layer is higher than that of the inner layer with the hot face in contact with molten iron.
A composite lining configuration, especially for the bosh and for the shaft zone of a blast furnace is disclosed for example in U.S. Pat. No. 3,953,007. This patent suggests two separate layers of different carbonaceous refractory material e.g. an outer layer of high-thermal conductivity graphite blocs and an inner layer of silicon-carbide having high resistance to wear and chemical attack. In fact, in the field of blast furnace refractories it is known to add silicon carbide or silicon metal to the carbonaceous mix in order to improve (reduce) permeability, reduce pore sizes and improve abrasion resistance.
Concerning the hearth wall lining more specifically, a similar layered approach is proposed in U.S. Pat. No. 3,520,526. This patent suggests providing two layers of substantially equal thickness, with the thickness of the outer layer being preferably from 0.8 to 1.2 times that of the inner layer. More specifically, U.S. Pat. No. 3,520,526 suggests that the radially outer layer, which is in contact with the cooling system, e.g. with the staves, should have a thermal conductivity that is substantially higher than that of the radially inner layer, in particular at least five times higher.
Durability of the refractory lining of the furnace hearth is a critical factor as regards campaign duration, since failure of the refractory lining is one of the most common reasons for premature shut-downs. Accordingly, in order to achieve the desired campaign duration, sophisticated refractory materials and configurations are state-of-the-art and related expenses are accepted. Required qualities are among others: good resistance to erosion by molten pig iron, good resistance to oxidation, low carburization dissolution rate, high mechanical strength and high thermal conductivity to maintain the hot face at a temperature as low as possible. Consequently, considering the total construction cost of a hearth, the refractory lining on its own may well make up more than two thirds (66%) of the total cost, i.e. exceed the cost of the steel shell and the hearth cooling system. Obviously, in case of a reline keeping the existing shell and cooling construction, the refractory constitutes an even more important proportion in total cost.
On the other hand, as is also well-known, there is an ongoing trend towards ever increasing production rates. The production capacity of a blast furnace is limited, among others but in notable manner, by the useful internal volume of the hearth, which volume is radially limited by the lining thickness and the shell diameter.
In view of the foregoing, there is obviously a desire for reducing the total wall thickness of the hearth wall lining so as to achieve either, or preferably both, of the benefits of reduced lining cost and increased useful internal volume of the hearth.